The compact size and high efficiency of semiconductor laser diodes make them the ideal candidates for applications requiring concentrated and spectrally pure laser light sources. Applications such as optical storage, low end printing and telecommunications that once used many different types of laser light sources, now almost exclusively use semiconductor diode lasers once these diode lasers with the required characteristics were successfully developed. The primary reason that semiconductor diode lasers have these very useful characteristics is that the excited or pumped laser area can be made very small through the use of semiconductor fabrication techniques such as photolithography and epitaxial layer growth. Due to the small lasing area, the gain and optical intensity, which are the two main ingredients necessary for efficient conversion of excited atoms in the lasing medium to lasing photons, the efficiency of a laser diode can be very high. This effect produces a laser source of high brightness: that is, a source of a certain power with relatively low beam divergence for its wavelength. Brightness can either be defined in terms of its Lagrange invariant, the area of the emitting light source times the solid angle of the divergence of the light from the source, or in the case of Gaussian beams, the M2 parameter, known as the beam quality factor. An ideal beam is usually diffraction-limited and has an M2 of 1, which is the lower limit for M2. Such beams with low M2 have a light intensity distribution that is substantially symmetric, viewed axially, with an ideal beam being perfectly symmetric.
Nevertheless, this primary advantage of semiconductor laser diodes—small lasing volumes—becomes a disadvantage when scaling these devices to higher powers. Single TEM00 mode operation near the diffraction limit requires lasing modal dimensions (laser diode stripe width) to be typically less than 3-5 microns. As the power extracted from these lasing dimensions is increased, optical facet damage and other power related damage mechanisms usually limit the available power from these devices to be less than 500 mW. As the laser diode stripe width is increased to about 100 microns, powers in excess of ten (10) Watts can be achieved but at much reduced beam quality. Substantial asymmetries are seen in the fast axis brightness versus the slow axis brightness. The output from single broad area diode lasers is significantly inadequate for many applications in terms of both power level and beam quality. For example, applications in the of high-power processing of materials such as welding and the cutting and heat treating of materials such as metals, require power levels in the range of approximately 1 kW to approximately 5 kW with beam qualities equivalent to the output of a approximately 200-400 microns by approximately 0.14-0.22 numerical aperture (NA) multimode optical fiber.
The need to scale the output of semiconductor laser diodes to higher powers while maintaining beam quality has led to several approaches. The first is the well understood and documented approach to use these laser diodes to pump a solid state gain material such as NdYAG. In this approach, a low brightness output beam of an incoherent semiconductor laser diode array is converted via the gain material to a TEM00 output beam having an M2 nearly as low as 1.0. Another approach is to fiber-couple the output of many individual laser diodes or laser diode bars and cladding-pump a rare-earth-doped fiber laser. Near diffraction-limited M2 values of <1.1 have been achieved with power levels greater than 800 W in an Yb doped double-clad fiber laser.
Applications such as material processing and solid state laser pumping require beams with M2 values near the diffraction-limit, and much attention has been given to the use of beam shaping and steering techniques to improving the quality of the stacked laser diode array bars themselves. Most of these efforts have focused on beam shaping and steering techniques that treat the laser bar emitter as a single wide source (greater than 19% fill factor) of 5 to 10 mm in width. Devices using these techniques producing approximately 600 Watts in a 600 μm 0.22 NA multimode optical fiber are commercially available. Earlier techniques utilize individual 100 μm-wide laser emitters. These use either individual laser diode emitters aligned with the devices oriented perpendicular to the epitaxially grown diode junction along an arc, or individual laser diode emitters aligned in a single bar (less than 21% fill factor) that are individually collimated and passed through a 90° image rotating prism such that their fast axis directions become co-linear. While the image rotation technique has yielded high brightness beams, its application has been limited to single laser diode bars, and the power that can be concentrated in a single beam is far short the power levels that are required for many applications. High power broad area emitters are also required for some telecommunications applications and high power bars with less than 21% fill factors have recently become available with lifetimes that are long enough to meet the requirements of industrial applications.